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Cylindrospermopsis raciborskii- dominated harmful algae blooms have been reported globally in recent years. However, our understanding of the ecology of C. raciborskii in natural conditions is still poor. In this study, we collected the water samples from a C. raciborskii -blooming lake, Yilong Lake, in Yunnan province, China, and used both culture-dependent and culture-independent approaches to investigate their microbial communities and the interactions between C. raciborskii and the other bacteria. The composition and diversity of microbial communities were revealed with 16S rRNA gene high-throughput sequencing data analysis. Microbial co-occurrences analysis suggests C. raciborskii may have complex associations with other bacteria. Based on co-inoculation tests, we obtained 14 strains of bacterial strains from the water samples that exhibited either algicidal or promoting effects on a strain of C. raciborskii. Two bacterial isolates exhibited a consistent performance between co-occurrence analysis and experimental results. Effects of these bacteria-algae interspecies interactions on the bloom event are discussed. All these results may provide new insights into the C. raciborskii -dominated blooms and how its interspecies relationships with other bacteria may influence the bloom events in eutrophic waters throughout the world. [ABSTRACT FROM AUTHOR]

Aims: Algicidal bacteria can be used for control of harmful algal bloom and extraction of algal bioproducts based on their algae‐lysing activities. This work investigated the algae‐lysing activity of a newly isolated algicidal bacterium, Paenibacillus polymyxa strain MEZ6 and its possible mechanisms. Methods and Results: Algicidal bacteria were isolated from soil samples collected at the university campus. Co‐inoculation tests identified that one isolate, MEZ6, can rapidly kill eukaryotic algae including Chlamydomonas reinhardtii, Tribonema minus, Haematococcus pluvialis, and Chlorella ellipsoidea. The strain was determined as Paenibacillus polymyxa MEZ6 based on 16S rRNA gene sequence and genome comparisons. The algicidal activity was detected in both living cells and cell‐free supernatant of spent culture medium, suggesting cell‐cell contact is not required for algicidal activity. Strain MEZ6 was less active towards cyanobacterial strains compared to algae. Genomic sequence and comparative proteomic analyses were performed to explore the possible algicidal mechanisms of the strain. Differentially expressed protein analysis identified a number of proteins related to polysaccharides degradation and antimicrobial secondary metabolite biosynthesis that may be involved in the algicidal activity of MEZ6. Conclusion: Paenibacillus polymyxa MEZ6 is a newly discovered gram‐positive algicidal bacterial strain with high lytic activity towards several algal species. Significance and Impact of the Study: Our study extends the understanding of the versatile characters of Paenibacillus polymyxa and sheds new insights into its application in algae biotechnology. [ABSTRACT FROM AUTHOR]